The present invention relates to an aluminum alloy sheet, which is improved in weldability, filiform corrosion resistance, bake-hardenability, and formability, and a method for manufacturing the same, and more particularly, to an aluminum alloy sheet, adapted for use as a material for the parts of land transportation vehicles, household electric appliances, various other machines, etc., which are formed by pressing or bending and used directly or after being assembled by welding, and a method for manufacturing the same.
Conventionally, aluminum alloy sheets, which are applicable to car components and other uses, are formed by pressing or bending. In some cases, they are subjected to paint baking (hereinafter referred to simply as baking), whereby they are heated after coating.
Presently, Al-Mg-Si alloys, such as 6009, 6010, 6111, and Al-Cu alloys, such as 2036, are practically used for aluminum alloy sheets which are adapted for forming and back hardening (process for improving the strength by baking). Although these alloys are poorer in formability than Al-Mg alloys, such as 5182, they are superior in bake-hardenability (susceptibility to baking). In particular, these specific alloys are characterized in that their strength can be improved by baking at a relatively high temperature of about 200.degree. C. The above numbers of four figures are international registered designations for wrought aluminum and wrought aluminum alloys.
In consideration of the recently available baking temperature which tends to be as low as 175.degree. C., the inventors hereof have proposed aluminum alloys adapted for this tendency, in Japanese Patent Applications Nos. 60-210768 and 61-18860.
For the application described above, weldability for assembly and corrosion resistance during use, as well as formability and bake-hardenability, are essential factors.
First, the weldability will be explained. In most cases, inner and outer panels are conventionally spot-welded together, in order to increase the rigidity of various parts of automobiles. Recently, however, aluminum alloy materials have started to be use not only for panel members but also for frame members. In other words, applicable regions for the aluminum alloy materials have increased. Thus, there have been increased places where the spot welding cannot be effected or cannot ensure satisfactory strength, or where fine cracks or the like, caused during the forming process, must be repaired. Recently, therefore, MIG or TIG arc welding have come to be frequently required in place of the spot welding.
Although the conventional alloys, such s the Al-Mg-Si alloys or the Al-Cu alloys, are poor in spotweldability, they have properties good enough to stand use. If subjected to arc welding, however, the Al-Cu alloys are liable to bead cracking, while heat-affected zones of the Al-Mg-Si alloys may sometimes suffer minor microcracks. Accordingly, the Al-Cu and Al-Mg-Si alloys are not suited for the application which requires arc welding.
The following is a description related to the corrosion resistance. Since anti-freezing mixtures, such a calcium chloride, may sometimes be sprinkled over roads, the corrosion resistance of car components is becoming a more important factor.
Aluminum alloys are superior to steel in corrosion resistance. If the paint comes off to expose the base material, however, filiform corrosion, a mode of corrosion different from normal corrosion, is caused at the boundary between the paint and the aluminum-alloy base material. The aforesaid Al-Cu and Al-Mg-Si alloys may also suffer filiform corrosion. To cope with this, therefore, the aluminum alloy sheet, as well as paints and the coating method (including a process for base treatment), has conventionally been improved. Heretofore, however, no satisfactory solution to the problem has been found yet.